Effect pigment platelets of iron, also referred to as iron flakes or iron effect pigments, are used in particular on account of their magnetic orientability in the decorative field. By orienting the iron pigments along the magnetic field lines in the application medium while it is still liquid, it is possible to generate very attractive 3D effects or holograms.
The production of platelet-shaped iron pigments by dry or wet grinding of reductively treated carbonyl iron powder in the presence of grinding assistants, with a size which is desired in particular for automotive finishing and with a “silver dollar” shape known from aluminum pigments, is described comprehensively in DE 101 14 446 A1, for example.
In this known grinding operation, the coarse powder particles are mechanically deformed into iron platelets, and are not comminuted. In order to prevent cold welding of the iron particles in the course of grinding, a lubricant is added, a fatty acid such as stearic or oleic acid, for example. These pigment platelets of iron, provided with an inhibitor coating, composed of a passivating anticorrosion coating and an inhibitor layer, have an average particle size of 5 to 100 μm and an average particle thickness of 500 to 30 nm. Although this specification describes the production of optically appealing effect pigments with high flop, such products have not been able to establish themselves on the market, owing to their optical properties. Thus, the Ferricon 200 product (Eckart) is sold only on the basis of its magnetic properties, which allow orientation of the pigment platelets in a magnetic field and, resulting therefrom, interesting 3D effects. As a normal effect pigment, however, acting on the basis of its plane-parallel orientation in an application medium, it has to date been unable to attract interest.
Relative to highly brilliant aluminum pigments, the hue was too matt and dark, owing to the lower reflectivity of iron.
In the printing ink sector as well, the pigment has to date been unable to find any application as a pure effect pigment. Particularly in so-called reverse-face application, it has provided no satisfaction to date. With PVD aluminum pigments, in contrast, proper mirror effects can be produced in reverse-face application.
On the other hand, there is a high demand in the market for black, lustrous metallic effect pigments. The iron effect pigments known from DE 101 14 446 A1 do not satisfy this demand, in relation to gloss and darkness or blackness.
DE 101 14 445 A1 also discloses iron effect pigments coated with metal oxides.
DE 10 2007 007908 A1 discloses dark, black metallic effect pigments. These pigments, however, are obtainable only via PVD processes, and a defined oxygen content must be established in the metallic effect pigments produced, thus making them very expensive and also costly and inconvenient to produce.
DE 103 15 775 A1 discloses aluminum pigments that are coated with fatty acid and optionally with a passivating inhibitor or anticorrosion layer, and which have an average thickness of 30 to 100 nm.
Lastly, EP 1 621 586 A1 describes a platelet-shaped aluminum pigment having a pronounced metal gloss and a fine grain distribution. This aluminum pigment has an average thickness of 0.025 to 0.08 μm and an average particle diameter (D50) of 8 to 30 μm.